Influenza viruses belong to Orthomyxoviridae and are classified into type A, type B and type C depending on the antigenicity of the nucleoprotein and the matrix protein located in the virus. The type A and type B viruses have been prevalent every year, and in particular, the type A virus is classified into 16 subtypes of haemagglutinin and 9 subtypes of neuraminidase depending on the glycoproteins which are surface antigens on the particles, and undergoes antigenic variation easily.
Therefore, the vaccine strain has to be selected based on the prediction of the prevalence in each season, and it is necessary to prepare a reagent for measuring haemagglutinin which is a main antigen of vaccine in accordance with the change of the strain.